Turbine blades, often referred to as buckets, are subject to vibrational stresses that can impact engine efficiency and part life. To reduce these stresses, a number of ways of damping or limiting bucket vibrations have been devised. One approach is to frictionally dampen certain modes of vibrations by interlocking the tips of covered or tip-shrouded buckets. To dampen vibratory stimuli and control natural frequencies, the integral covers or shrouds of the buckets must maintain contact from bucket to bucket within an annular row. To create the requisite interlock, the airfoil or blade portions are twisted during assembly. This pre-twist is in a circumferential direction as viewed along the long axis of the respective bucket. During operation, centrifugal forces will cause radial growth and twisting of the bucket blade portions, tending to open circumferential gaps between the blade tip covers. Thus, the covers must be assembled with enough compressive contact force between the respective adjacent buckets to provide residual force during operation despite the effects of centrifugal forces. The greater the interference required, the greater the required angle of rotation.
In other words, the present method of assembling integrally covered buckets is to twist the airfoil portion of each bucket so that the pitch of the tip cover (or simply, “cover”) decreases, allowing an entire row of buckets to be placed on the rotor. The inherent torque of the airfoil portion then causes the cover to untwist which produces a residual interference that keeps the row of buckets coupled during operation.
The torque characteristics of the airfoil portion of the bucket may preclude the use of an integral tip cover, however, if the torque characteristics of the airfoil portion do not provide for the desired coupling face pressure at the integral bucket tip covers.